1. Field of the Invention
This invention relates to a hydrophilicizing treatment for a surface with underlying metal, normally one already bearing a corrosion protective conversion coating such as a chromate or phosphate conversion coating. After treatment according to this invention, the surface will have a tendency for water to spread spontaneously over the surface. The invention is particularly advantageously applicable to provide aluminum evaporators, heat exchangers, and condensers with hydrophilic coatings that have good corrosion resistance, especially to sea water, and a durably hydrophilic character with little or no tendency to develop undesirable odors during use.
2. Statement of Related Art
Although any of the common structural metals can be used in constructing practical heat exchanging surfaces, aluminum and its alloys are among those most often used, because of their high heat conductivity. In heat exchanger surfaces, metals are normally used without any relatively thick protective coating such as a paint or lacquer that would normally be used to improve the resistance of the metals to corrosion in most other types of metal structures, but is avoided in heat exchangers because such a coating would also reduce the efficiency of heat exchange.
During the cooling of hot air, a common use of these heat exchangers, moisture contained as vapor in the hot air condenses and initially forms water drops or beads on the fins of the heat exchanger. If the surface of the heat exchanger fins is not sufficiently hydrophilic, the water beads accumulate on the fin surface and impede the air flow between fins and also reduce the heat transfer efficiency. The condensed water beads also tend to absorb dust and contaminants in the air, such as carbon dioxide, nitrogen oxides, and sulfur oxides, which often promote aluminum corrosion. Therefore, the formation of water beads on the fins of aluminum heat exchanger not only decreases heat transfer efficiency but also can cause the corrosion of aluminum.
In order to achieve a desirable combination of a hydrophilic nature and corrosion resistance on metal, particularly aluminum, surfaces, various coatings and treatments have been tried, but no perfectly satisfactory result has yet been achieved. This is particularly true for uses in which the hydrophilicized surface comes into contact with sea water, which is especially corrosive, so that currently available hydrophilicizing treatments leave aluminum and its alloys with often inadequate corrosion protection.